1. Field of the Invention
This invention relates to light emitting diodes and more particularly to a light emitting diode apparatus and method for efficiently generating light from a wavelength down-conversion material.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light and generally comprise an active region of semiconductor material sandwiched between two oppositely doped layers of semiconductor material. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED. Recent advances in LEDs (such as nitride based LEDs) have resulted in highly efficient light sources that surpass the efficiency of filament based light sources while providing a light with equal or greater brightness in relation to its input power.
One disadvantage of conventional LEDs used for lighting applications is that they cannot generate white light from their active layers. One way to produce white light from conventional LEDs is to combine different colors from different LEDs. For example, white light can be produced by combining the light from red, green and blue LEDs or combining the light from blue and yellow LEDs. One disadvantage of this approach is that it requires the use of multiple LEDs to produce a single color of light, increasing the overall cost and complexity. Different colors of light are also often generated from different types of LEDs, and combining different LED types on one device can require complex fabrication and can require different control voltages. These types of emitters are complicated and costly to manufacture since each device may have different electrical requirements and may behave differently under varied operating conditions (e.g. with temperature, current or time).
The spectral emission lines of component LEDs are typically narrow (e.g. 10-30 nm FWHM) and wavelength ranges exist in which it is difficult to obtain high-efficiency LEDs (e.g. approximately 550 nm). As a result, it may be difficult to achieve both high efficacy and high color rendering index with low cost and high yield. This can be particularly problematic when spectral requirements call for high efficiency green LEDs, since such LEDs have only been realized in the (In, Ga, Al)N system and are typically subject to low efficiency and wavelength and emission variations with operating conditions such as drive current and temperature. While more simplified white lamps may be realized using only two LEDs emitting at complimentary colors (e.g. blue, yellow), it is exceedingly difficult to achieve high color rendering coefficients in such lamps.
Light from a single blue emitting LED has also been converted to white light by surrounding the LED chip with a yellow phosphor, polymer or dye, with a typical phosphor being Ce:YAG. [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding material “down-converts” the wavelength of at least some of the LED light, changing its color. For example, if a nitride based blue emitting LED is surrounded by a yellow phosphor, some of the blue light will pass through the phosphor without being changed, while the remaining light will be down-converted to yellow. The LED will emit both blue and yellow light, which combines to provide a white light.
One conventional method for coating an LED with a phosphor layer utilizes a syringe or nozzle for injecting a phosphor containing epoxy over the LED. Another conventional method for coating an LED is by stencil printing, which is described in European Patent Application EP 1198016 A2 to Lowery. Multiple light emitting semiconductor devices are arranged on a substrate with a desired distance between adjacent LEDs. The stencil is provided having openings that align with the LEDs, with the holes being slightly larger than the LEDs and the stencil being thicker than the LEDs. A stencil is positioned on the substrate with each of the LEDs located within a respective opening in the stencil. A composition is then deposited in the stencil openings, covering the LEDs, with a typical composition being a phosphor in a silicone polymer that can be cured by heat or light. After the holes are filled, the stencil is removed from the substrate and the stenciling composition is cured to a solid state.
Another conventional method for coating LEDs with a phosphor utilizes electrophoretic deposition. The conversion material particles are suspended in an electrolyte based solution. A plurality of LEDs are arranged on a conductive substrate that is then almost completely immersed in the electrolyte solution. One electrode from a power source is coupled to the conductive substrate at a location that is not immersed in the solution, and the other electrode is arranged in the electrolyte solution. The bias from the power source is applied across the electrodes, which causes current to pass through the solution to the substrate and its LEDs. This creates an electric field that causes the conversion material to be drawn to the LEDs, covering the LEDs with the conversion material.
In these conventional LED packages the phosphor conversion elements are in close proximity to the LED chip, and due to scattering and Lambertian emission characteristics of phosphors, a significant portion of the light is emitted back into the LED package and LED chip. Upon transmission through the phosphor layer, photons can also experience multiple scattering events. Light, which is scattered or emitted by the phosphor layer and directed toward the LED chip, is subject to the light extraction limitations of the chip. The LED chip light extraction efficiency is typically less than unity, resulting in further photon losses. Light that is scattered back into the package can be reflected back, but the reflectivity of package materials is typically only 70-90%. The scattering and back emission increases the path length for the emitted light within the package and thus the chance to experience losses.